The present invention relates to a centrifugal pump for pulp suspensions of lignocellulosic material. More particularly, the present invention relates to a pump where a gas, such as air, is separated from the pulp in the pump.
Gas which follows along with the pulp can cause significant problems during pumping with centrifugal pumps. The gas in the pulp collects in the form of bubbles, which accumulate in front of the impeller whereby the capacity of the pump deteriorates. This problem can be overcome by evacuating the gas from the space in front of the impeller, such as by means of vacuum pumps of the type comprising liquid ring pumps. These pumps often comprise a separate vacuum pump, whose suction line is connected to the sealing space of the pulp pump. These pumps can also be designed as a separate vacuum impeller in the form of a liquid ring impeller located on the shaft of the pulp pump so that a common drive for both impellers is obtained.
In the cases when a separate vacuum pump is used, the pulp pump becomes relatively complicated and expensive, due to the fact that several components are required in the system. When, a separate liquid ring impeller of a conventional type is placed on the same shaft as the pulp pump, the capacity of the liquid ring impeller is restricted for reasons of space.
In accordance with the present invention, these prior art pumps have been improved upon by the discovery of a centrifugal pump for pumping a pulp suspension comprising a housing including a housing inlet for the pulp suspension and a housing outlet for the pumped pulp suspension, a rotatary shaft within the housing, an impeller including a hub and a plurality of blades radially extending from the hub, the impeller mounted for rotation on the rotary shaft, a wheel disk mounting the plurality of blades, the wheel disk including a front face facing the hub and a rear face, the wheel disk including openings for discharging gas generated in the pump, a plurality of vacuum blades mounted for rotation on the hub behind the rear face of the wheel disk, a vacuum housing enclosing the plurality of vacuum blades, the vacuum housing including an outer cylindrical wall radially displaced from the plurality of vacuum blades, the outer cylindrical wall being eccentrically disposed with respect to the plurality of vacuum blades, the vacuum housing further including a front wall proximate to the rear face of the wheel disk, and a rear wall distal from the rear face of the wheel disk and separated from the plurality of vacuum blades by a gap, the front wall including a suction opening for receiving the gas, the rear wall including an exhaust opening for exhausting the gas, the rear wall being axially adjustable whereby the gap between the plurality of vacuum blades and the rear wall can be adjusted in order to adjust the vacuum generated thereby, and a passageway disposed between the rear face of the wheel disk and the front wall for communication between the openings in the wheel disk and the suction opening. Preferably, the front wall includes at least one aperture and the pump includes a dilution chamber disposed radially outwardly with respect to the outer cylindrical wall for supplying dilution liquid to the passageway through the at least one aperture. In a preferred embodiment, the pump includes a shaft seal for the rotary shaft, the shaft seal defining a space axially adjacent to the rear wall, and the dilution chamber includes an opening in communication with the space, whereby the dilution liquid can externally flush the shaft seal.
In accordance with one embodiment of the centrifugal pump of the present invention, the pump includes a connector disposed adjacent to the vacuum chamber, whereby a separate vacuum system can be coupled to the centrifugal pump.
In accordance with the present invention, the aforesaid problems can be eliminated by the vacuum impeller being integrated in the impeller of the pulp pump. The structure of the present invention is compact and simple, and the capacity of the vacuum pump can be easily controlled. The pump is thus formed so that it can easily be combined with a separate vacuum pump, so that both pumps can be used in parallel. Alternatively, the separate vacuum pump can be coupled in at operation drop-out of the integrated vacuum pump according to the present invention in order to ensure accessability for the centrifugal pump.